Recent Advances in Multicomponent Organic Composite Thermoelectric Materials

Recent Advances in Molecular Design of Organic Thermoelectric Materials

Thermoelectric materials have attracted more attention in recent years, which can be corroborated by the increasing scientific publications. Moreover, the optimistic prediction for the thermoelectric industry proves that the practicability of thermoelectric technology is further acknowledged. Recently, benefitting from the rapid development of organic electronics, the research of organic thermoelectric (OTE) materials is receiving particular interest. Cooperation and complementation between organic and inorganic thermoelectric materials could promote the broader application of thermoelectric effect. To realize high conversion efficiency of thermoelectric device, high‐performance p‐ and n‐type OTE materials are both necessary. However, the instability of most n‐type organic materials in air impedes their application for high‐performance thermoelectric conversion. Therefore, more efforts should be made to promote relevant research and applications. Herein, the research progress on OTE materials (n‐type) and devices is reviewed to show readers some details of n‐type OTE research and give some guidelines for further explorations.

Organic thermoelectric (TE) materials can directly convert heat to electricity, and they are emerging as new materials for energy harvesting and cooling technologies. The performance of TE materials mainly depends on the properties of materials, including the Seebeck coefficient, electrical conductivity, thermal conductivity, and thermal stability. Traditional TE materials are mostly based on low-bandgap inorganic compounds, such as bismuth chalcogenide, lead telluride, and tin selenide, while organic materials as promising TE materials are attracting more and more attention because of their intrinsic advantages, including cost-effectiveness, easy processing, low density, low thermal conductivity, and high flexibility. However, to meet the requirements of practical applications, the performance of organic TE materials needs much improvement. A variety of efforts have been made to enhance the performance of organic TE materials, including the modification of molecular structure, and chemical or electrochemical doping. In this review, we summarize recent progress in organic TE materials, and discuss the feasible strategies for enhancing the properties of organic TE materials for future energy-harvesting applications.

Thermoelectric (TE) material, as one of new energy materials, is regarded as one of the most important energy-saving materials, which can directly achieve the interconversion between heat and electricity. Presently, inorganic semi-conductors are considered to be the best thermoelectric materials. However, the development of new thermoelectric material has attracted great attention owing to the scarce resource and limited performance of inorganic thermoelectric materials. As the discovery and wide application of conducting polymers (CPs), organic thermoelectric materials have come into the sights of people over the past 30 years. The development of CPs as a promising thermoelectric material has gained great attention over the past decades. Various CPs have been developed and investigated on thermoelectric performance such as polyacetylene (PA), polyaniline (PANi), polypyrrole (PPy), polythiophene (PTh) and their derivatives since 1989. Among numerous CPs, poly(3,4-ethylenedioxythiophene) (PEDOT) shows many superior properties compared to others due to its excellent environmental stability, water solubility, easy processibility, and high electrical conductivity, which brings new strategy for studies of high performance organic thermoelectric materials. The conductive PEDOT combined with an insulating poly(styrenesulfonate) (PSS) can form a stable aqueous PEDOT:PSS with a good film-forming property and has been regarded as one of the most potential thermoelectric material. In 2008, the thermoelectric performance of PEDOT:PSS pellets were reported systematically for the first time. Although its thermoelectric figure-of-merit ( ZT ) was as low as 10 - 3, it was one of the highest values for CPs at that time. Soon afterwards, the thermoelectric performance of the free-standing PEDOT:PSS film achieved a ZT value as high as 10 - 2 in 2010. During these years, great attention focused on the derivatives of PTh, polyselenophene (PSh), PANi, and polycarbazole (PCz). Since 2010, PEDOT:PSS has come into sight of researchers in the world. The past few years witnessed great development of thermoelectric performance of conducting PEDOT:PSS ( ZT ~10 - 1). In recent ten years, the thermoelectric figure-of-merit ( ZT ) of PEDOT has been enhanced by three orders of magnitude from 10 - 4 to 10 - 1 as one of the most promising organic thermoelectric materials. Presently, the enhanced thermoelectric performance for PEDOT concern in the increased electrical conductivity via an easy method, especially for PEDOT:PSS. A large electrical conductivity of PEDOT:PSS thin film more than 3000 S cm - 1 has been achieved by adding an organic solvent or post-treatment with organic solvents, acid, and ionic liquids. Compared to inorganic thermoelectric materials, PEDOT:PSS can achieve a high electrical conductivity without an obviously decreasing Seebeck coefficient. A further improvement of thermoelectric performance for PEDOT:PSS has been devoted to the optimization of Seebeck coefficient via the pH value adjustment, the reduction of the oxidized level of conductive PEDOT, or composite with inorganic thermoelectric materials. A large thermoelectric power factor has become a reality. Although there are large gaps from actual industrial application for thermoelectric PEDOT ( ZT ˃1), yet most efforts focus on the high performance PEDOT. A large number of new techniques and methods have been developed to improve the thermoelectric performance of PEDOT. This review pays the attention to the advantages and characteristics, development history, and performance improvement of PEDOT as a promising organic thermoelectric CP from discovery to development. In order to achieve a high performance thermoelectric PEDOT, more attention should be paid to the development of low dimensional PEDOT crystal, control of oxidized level, extension of conjugated chain length of PEDOT, new preparation method and techniques, and the effects of crystal structure on electron transport properties as well as the flexible PEDOT thermoelectric devices in the future. Additionally, it is necessary to keep up with the development of n-type organic thermoelectric materials.

Nowadays, organic thermoelectric (TE) materials have attracted considerable attention due to their unique merits, e.g., light‐weight, high mechanical flexibility, nontoxicity, easy availability, and intrinsically low thermal conductivity. Among the organic/polymer TE materials reported so far, poly(3,4‐ethylenedioxythiophene):poly(styrenensulfonate) (PEDOT:PSS) is extensively investigated because it is water‐processable, thermally stable, and can be highly conductive. Over the past few years, the TE properties of the PEDOT‐based TE materials are continuously improved. With rational design, some PEDOT:PSS‐based materials have achieved high ZT values comparable to the conventional inorganic TE materials like bismuth telluride at room temperature. This paper reviews the recent breakthroughs for PEDOT:PSS‐based TE polymers and composites. The strategies for achieving high‐performance PEDOT:PSS‐based TE materials and the corresponding underlying mechanism are specifically discussed. The TE devices fabricated by the PEDOT:PSS‐based TE materials are also presented, in terms of their fabrication/assembly technique, device configuration and device performance. With all the exciting progress made in the PEDOT:PSS‐based TE materials, the further development and practical applications of the high‐efficient organic TE materials as flexible TE module devices and wearable electronics can be greatly anticipated.

Chapter 9 - Synthesis and Processing of Thermoelectric Nanomaterials, Nanocomposites, and Devices

Thermoelectric (TE) materials possess unique energy conversion capabilities between heat and electrical energy. Small organic semiconductors have aroused widespread attention for the fabrication of TE devices due to their advantages of low toxicity, large area, light weight, and easy fabrication. However, the low TE properties hinder their large-scale commercial application. Herein, the basic knowledge about TE materials, including parameters affecting the TE performance and the remaining challenges of the organic thermoelectric (OTE) materials, are initially summarized in detail. Second, the optimization strategies of power factor, including the selection and design of dopants and structural modification of the dope-host are introduced. Third, some achievements of p- and n-type small molecular OTE materials are highlighted to briefly provide their future developing trend; finally, insights on the future development of OTE materials are also provided in this study.

Energy demand and importance are increasing with the industrial development and the development of high technology in modern society. However, the global supply of energy is facing problems such as fossil energy depletion and environmental pollution, and research and interest in alternative energy are increasing. Among them, research on thermoelectricity is being actively conducted. The thermoelectric characteristics are represented by the Seebeck coefficient, electrical conductivity, and thermal conductivity at the operating temperature. This is expressed as a thermoelectric performance index (ZT). ZT appears in proportion to the electrical conductivity and Seebeck coefficient, and inversely proportional to the thermal conductivity. Thermoelectric materials are divided into inorganic thermoelectric materials and organic thermoelectric materials, both of which have advantages and disadvantages. The inorganic thermoelectric material has high thermoelectric performance, but it is highly toxic and has low electrical conductivity. On the other hand, organic thermoelectric materials have lower performance than inorganic materials they are easy to obtain and have high electrical conductivity. In this study, organic-inorganic hybrid thermoelectric material was studied by combining the advantages of each material.In this study, Te nanowires (NWs) were synthesized at 50 Co by galvanic replacement reaction. We used Si (1 0 0) wafer as substrate of NWs. The substrate was degreased with acetone and ethanol. The solution of cadmium(1M), hydrofluoric acid (HF 51.0%, 4.5M) and tellurium dioxide (TeO2, 2mM). After the reaction, the synthesized Te NWs were washed with deionized water and IPA. The prepared Te NWs reacted with solution (silver nitrate in ethylene glycol) for topotactic reaction. After the reaction, the wires are washed several times with ethanol. The hybrid TE was fabricated by coating PEDOT:PSS on Ag-Te NWs with different ethylene glycol (EG) doping.A composite thermoelectric material was manufactured using Te (inorganic thermoelectric material) and PEDOT:PSS (organic thermoelectric material). Ag-topotactic reaction was conducted to increase the electrical conductivity of NWs. As the EG doping concentration of PEDOT increased, the electrical conductivity because the layer of PEDOT:PSS became thinner and didn’t fill the gap of NWS sufficiently. In conclusion, the Te NWs coated with PEDOT:PSS with 10% EG doping showed the best electrical conductivity and power factor. The more details will be presented. Figure 1

Polymeric thermoelectric PEDOT: PSS & composites: Synthesis, progress, and applications

Contemporary advances in organic thermoelectric materials: Fundamentals, properties, optimization strategies, and applications

Organic thermoelectric (OTE) materials have been regarded as a potential candidate to harvest waste heat from complex, low temperature surfaces of objects and convert it into electricity. Recently, n-type conjugated polymers as organic thermoelectric materials have aroused intensive research in order to improve their performance to match up with their p-type counterpart. In this review, we discuss aspects that affect the performance of n-type OTEs, and further focus on the effect of planarity of backbone on the doping efficiency and eventually the TE performance. We then summarize strategies such as implementing rigid n-type polymer backbone or modifying conventional polymer building blocks for more planar conformation. In the outlook part, we conclude forementioned devotions and point out new possibility that may promote the future development of this field.

Organic thermoelectric materials have emerged as compelling candidates for harvesting low‐grade heat in flexible and lightweight energy systems. Compared to conventional inorganic thermoelectric materials, organic thermoelectric materials offer distinct advantages, including intrinsically low thermal conductivity, mechanical flexibility, and compatibility with large‐area and solution‐based processing. While p‐type materials such as poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) have been extensively optimized through solvent treatments and de‐doping strategies, recent advances in air‐stable n‐type polymers such as poly(benzodifurandione) (PBFDO) have greatly narrowed the performance gap and made it feasible to construct fully organic thermoelectric modules. This review highlights recent progress in organic thermoelectric materials with a focus on molecular design, doping mechanisms, and device‐level integration. We examine how novel polymers, dopant formulations, and emerging concepts have been driving improvements in the performance of organic thermoelectric materials toward practical application. Our group's previous contributions to module design such as thermal lamination techniques and integrated circuits are presented as case studies of system‐level implementation. Despite their relatively modest power factors and thermoelectric figures of merit, organic thermoelectric materials possess unique advantages in terms of low weight, processability, and scalability that make them especially suited for gram‐scale modules and powering small‐scale electronic devices and Internet‐of‐Things systems using ambient thermal energy.

Development of chemically doped high performance n-type organic thermoelectric (TE) materials is of vital importance for flexible power generating applications. For the first time, bismuth (Bi) n-type chemical doping of organic semiconductors is described, enabling high performance TE materials. The Bi interfacial doping of thiophene-diketopyrrolopyrrole-based quinoidal (TDPPQ) molecules endows the film with a balanced electrical conductivity of 3.3 S cm(-1) and a Seebeck coefficient of 585 μV K(-1) . The newly developed TE material possesses a maximum power factor of 113 μW m(-1) K(-2) , which is at the forefront for organic small molecule-based n-type TE materials. These studies reveal that fine-tuning of the heavy metal doping of organic semiconductors opens up a new strategy for exploring high performance organic TE materials.

Recent years have witnessed significant advancements in organic thermoelectric material systems, marked by rapidly expanding diversity, which provides a broad material selection platform for organic thermoelectric generators (OTEGs). However, effectively leveraging the intrinsic thermoelectric properties of organic materials through rational material design and device structure optimization remains a central challenge in this field. This work addresses this challenge by employing two easily fabricable organic thermoelectric composite materials: polyaniline/carbon nanotubes doped with camphorsulfonic acid (PANI/CNTs:CSA, p-type) and carbon nanotubes functionalized with polyethylenimine (CNTs:PEI, n-type). These composites were deposited onto flexible cotton fabrics via screen-printing technology and subsequently rolled into a high-thermoelectric-density rolled-type thermoelectric generator (TEG). Under a stable temperature difference (ΔT) of 50 K, the device exhibited favorable thermoelectric conversion performance, achieving an open-circuit voltage of 14.5 mV. Through finite element simulation modeling, this work for the first time established a three-dimensional multiphysics model capable of accurately describing the complex thermoelectric coupling behavior in rolled-type devices, systematically revealing the dynamic coupling mechanism between heat transfer and potential distribution. This approach clarified the intrinsic relationships among the heat source temperature, winding radius, and optimal height required to maximize device output power, thereby establishing universally applicable optimization guidelines. This work provides a theoretical foundation and practical structural optimization strategies for designing rolled-type organic TEG with a higher energy output density, significantly advancing their application in microelectronic self-powered systems.

Doping is essential to manipulate the electrical performance of both thermoelectric (TE) materials and organic semiconductors (OSCs). Although organic thermoelectric (OTE) materials have experienced a rapid development over the past decade, the chemical doping of OSCs for TE applications lags behind, which has limited further breakthroughs in this cutting-edge field. Recently, increasing efforts have been devoted to the development of energetically matched host and dopant molecules, exploring novel doping methods and revealing the doping mechanisms. This tutorial review covers the basic mechanisms, fundamental requirements, recent advances and remaining challenges of chemical doping in OSCs for TE applications. We first present the basic knowledge of the trade-off relationship in TE materials and its critical requirements for doped OSCs, followed by a brief introduction of recent advances in the molecular design of OSCs and dopants. Moreover, we provide an overview of the existing categories of doping mechanisms and methods, and more importantly, emphasize the summarized doping strategies for the state-of-the-art OTE materials. Finally, challenges and perspectives on the chemical doping of OSCs are proposed to highlight the research directions that deserve attention towards a bright future of OTE materials.